DC-to-DC power converters are widely used to supply power to electronic devices, such as computers, printers, etc. Such DC-to-DC converters are available in a wide variety of configurations for many different applications. For example, a relatively sophisticated DC-to-DC converter in integrated circuit form is available from Harris Corporation, the assignee of the present invention, under the model number HIP 6017. In fact, the HIP 6017 monitors and precisely controls three different output voltage levels for use in high performance microprocessor and computer applications.
The HIP 6017 integrated circuit includes a pulse width modulation (PWM) controller, a linear regulator, and a linear controller to provide the three outputs. In addition, the HIP 6017 integrated circuit includes the associated monitoring and protection functions. The PWM controller regulates the microprocessor core voltage with a synchronous-rectified buck converter. The linear controller regulates power for the so-called "GTL bus" or microprocessor bus of the computer, and the linear regulator provides power for the clock driver circuits.
A typical integrated linear regulator circuit, such as provided by one portion of the HIP 6017, is connected to and controls an external semiconductor pass device, such as a metal-oxide field-effect transistor (MOSFET). The MOSFET, for example, may be controlled to provide a regulated lower output voltage, such as 1.5 V, from a higher voltage input, such as 3.3 V. The gate of the MOSFET is driven by an error amplifier. The error amplifier is connected to the bias supply, such as 12 V for a typical computer application.
The error amplifier also has an inverting input connected to a resistive voltage divider for sensing the output or load voltage, and a non-inverting input connected to a voltage reference. For this example using a MOSFET pass device, the error amplifier needs to supply a control voltage in a range of about 0 to 12 volts, but at a relatively small current in a range of only about 5 milliamps regardless of the load current.
Other linear regulators are also used wherein an external NPN bipolar pass device is used as the pass device. In such a regulator, the error amplifier, more particularly the output stage of the error amplifier, need only supply a relatively modest voltage of about 0 to 2.5 V, for example. However, the current necessary to drive the base of the external NPN transistor is much higher than for the MOSFET pass device when the output load current is high. For example, with an output load current of 2 to 10 amps, the base drive current may be up to 50 mA.
If the linear regulator including the error amplifier coupled to the 12 V supply is used to drive the NPN bipolar device, the power dissipation on the integrated circuit will be 50 mA.times.12 V or 0.6 Watts. This is a relatively large amount of power, especially considering that there are typically other power dissipating circuits on the same integrated circuit chip, such as is the case for the HIP 6017, for example. In comparison, when the linear regulator is connected to an external MOSFET pass device, the power dissipation is only 5 mA.times.12 V or only 60 mW. Moreover, there are significant reasons why the circuit designer may wish to use the NPN bipolar transistor as the pass device rather than the MOSFET. Unfortunately, present linear regulator circuits that are also compatible for MOSFET pass devices may produce an unacceptably high power dissipation, because drive current is delivered from a relatively high supply voltage for compatibility with the MOSFET pass device.